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Old 22-06-2011, 12:10   #61
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Re: AIS reception issue

If you want to check out the AIS antenna and you can swap with the VHF antenna--don't buy a bunch of expensive test gear, just tune the VHF to a distant weather radio channel and swap antennas.

My AIS transponder checks for bad SWR, and will alert you via error codes or AISpro if it thinks its antenna is bad.
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Old 22-06-2011, 12:49   #62
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Re: AIS reception issue

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Originally Posted by s/v Jedi View Post
Not trying to be the smart a$$ here but actually... it isn't. I think somebody already mentioned that a dummy-load gives a perfect 1:1 SWR reading combined with a total failure to communicate. I have seen many cases where a 1:1.5 - 1:2 SWR was shown while there was no electrical connection between radio and antenna (just moisture in coax).
Nick makes an excellent point here.....
In addition to mositure in cables, as we know if twistedtree happens to have an "short" in his coaxial splice at 1/4 wavelength of coax (or odd multiples of such) from the SWR meter, he'd show a decent SWR but still may have serious reception/transmission problems......(or an "open circuit" at 1/2 wavelength / multiples)......not to mention other issues caused by intermittent RF connections, etc.....



And, FYI, yes lab-grade TDR's are still a bit pricey....but there are many portable, even pocket-sized, battery powered TDR's / cable fault detectors (I own one myself) that allow you to input the cables vel factor, and instantly see a digital display of the cable's length and position of opens/shorts, as well as cable impedance, etc....They cost about what you'd spend on really nice dinner out on the town....

And some even have LCD screens showing a sweep of the cable......
If want to spend about $1k, my favorite portable TDR is the AEA 20/20 TDR.....
Time Domain Reflectometer (TDR) - Cable Fault Locator - 20/20 TDR - - AEA Technology Inc.

Sorry to drift off topic...



Oh, and after you fix the coax "splice" (yes a PL-258 "barrel" and a couple of PL-259's is the right way to do it), or even better, replace the entire run of cable with one new/known good piece (as SoonerSailor mentioned), I agree with Don Radcliffe.....don't waste you $$$ on ant analyzers / TDR's etc.....use what you've got (multiple radios and multiple antennas) to determine whether you've got an antenna issue or not....




John
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Old 22-06-2011, 13:48   #63
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Re: AIS reception issue

Your multimeter should pick up any complete opens or shorts. This is the analyzer/TDR I use, its a bit cheaper than $1000:
AA230Pro

You might even find someone at the local ham club who would love to help and have an excuse to justify all the test gear he has accumulated....

I hope you're not put off by all of our "over-analyzing", but between the multimeter and the SWR meter you stand a good chance of finding the problem - assuming its not a soggy coax!
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Old 22-06-2011, 15:03   #64
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Re: AIS reception issue

I'm not claiming that SWR is the only measurement to take, since as has been pointed out a highly lossy cable approximates a dummy load -- the SWR looking into one end of the cable will be near 1:1 regardless of the load at the other end.

But remember, the SWR meter can also be used at both ends of a cable, and the relative forward power measurements will give you an indication of loss in the cable. Or (if your transmitter can handle the mismatch) you can measure the SWR with the far end of the cable disconnected from the antenna. A high SWR is good, and a low SWR is telling you that the cable is lossy. SWR meters are quite useful!
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Old 23-06-2011, 12:46   #65
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Re: AIS reception issue

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Originally Posted by ka4wja View Post
In addition to mositure in cables, as we know if twistedtree happens to have an "short" in his coaxial splice at 1/4 wavelength of coax (or odd multiples of such) from the SWR meter, he'd show a decent SWR but still may have serious reception/transmission problems......(or an "open circuit" at 1/2 wavelength / multiples)
I apologize for being pedantic, but this isn't correct. Assuming a low-loss cable, a short or open will give you a high (bad) SWR at any length. The impedance will vary from low to high as a function of cable length, but there will always be a reactive component that will result in a high SWR.

For a 1/4 wavelength cable, a short will look like an open, and an open will look like a short. With a 1/2 wave cable a short looks like a short, and an open looks like an open.

I used to design microstrip (printed circuit board) matching networks using shorted and open stubs -- this stuff is almost like magic.

Twistedtrees -- why are you slacking off? You've got tests to run and we demand a report! Get busy man!
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Old 23-06-2011, 13:27   #66
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Re: AIS reception issue

Paul,
Yep....you're correct.....
I was thinking of some old stubs/traps.....
I was at a client's office yesterday.... supposed to be working.....and ended up goofing off on-line for a while....
That's what you get...bad info...

Sorry about that....

John
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Old 24-06-2011, 05:19   #67
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Re: AIS reception issue

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Twistedtrees -- why are you slacking off? You've got tests to run and we demand a report! Get busy man!
It's too much fun watching you guys debate the behavior of defects in different length cables :-)

But in reality, I just got back from a trip to London and now need to go help my daughter move. I won't get back to the boat until next weekend, I'm afraid. I'm experiencing the "semi" in semi-retired.

But do carry on the debate. Perhaps a discussion of what the spice model looks like for one of these Shakespeare splice connectors? A random number generator?
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Old 01-07-2011, 16:29   #68
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Problem solved

As many were beginning to suspect, it turns out this was the most mundane of problems with the most bizarre symptoms/behavior. The only remaining question is how I ever picked up any AIS targets at all.

Drum roll, please....

The problem was the Shakespeare solder-less vampire cable splice device. I've got a dead short between the conductor and shield inside that coupler.

Using my VHF radio and antenna as a baseline, I measures forward power of 20W and reverse power of .5W with an SWR of about 1.35. Switching to the AIS antenna, I got forward power of 20W and reverse power of 10W! SWR was off the chart. I pulled the cable, cut out the splice, and measured a dead short between the shield and center conductor. What a piece of turd.

Everything is rebuilt now with PL-259s, fully soldered, and a barrel coupler. SWR is now about 1.4, and I'm picking up AIS targets everywhere. I switched on all my other electronics and all the targets remain on screen rock solid.

It's a small miracle that I ever picked up any targets at all with that splice connector in place, and no wonder that the slightest change of anything anywhere caused targets to disappear.
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Old 01-07-2011, 17:32   #69
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Good work in finding and fixing your problem. Be sure to wrap your connections at the barrel connector with CoaxSeal or some other self-vulcanizing tape to completely seal it off from the insidious maritime environment.

Chip
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Old 01-07-2011, 18:40   #70
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Re: AIS reception issue

Glad you found it and thanks for the feedback. Guess I'm going to have to send those beans after all...
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Old 01-07-2011, 20:38   #71
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Re: AIS reception issue

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Glad you found it and thanks for the feedback. Guess I'm going to have to send those beans after all...


ciao!
Nick.
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Old 02-07-2011, 18:21   #72
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Re: AIS Reception Issue

Twistedtree, that's great news! This also reminds me why I tend to ignore the 1/2 dB losses that some of my friends obsess about. It all eventually adds up, but so often it just doesn't matter. Now a shorted connector? That matters!
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Old 03-07-2011, 04:57   #73
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Re: AIS Reception Issue

By the way, I want to thank everyone for all the help sorting this out. I especially enjoyed the trip down memory lane as your guidance surfaced long-ago memories from my college AC circuits classes.
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